The present invention relates to a hardfacing composition, as well as an article having a hardfacing deposit. More particularly, the invention pertains to a hardfacing composition that is typically applied via a hardfacing rod, as well as an article having a hardfacing deposit, wherein the hardfacing deposit exhibits a microstructure that has improved consistency, as well as improved properties including wear properties such as, for example, abrasion resistance and erosion resistance.
Earth-engaging tools such as, for example, a rotary cone rock bit, typically operate in environments that subject the tools to wear such as erosive wear and abrasive wear. In order for such tools to function in a satisfactory manner, it is important for them to be able to resist wear including erosion and abrasion.
Heretofore, it has been known to deposit a hardfacing on the surface of an article (or substrate) whereby the hardfacing imparts improved properties, and especially wear properties including erosion resistance and abrasion resistance, to the article. Exemplary articles include an earth boring bit (e.g., a steel tooth rolling cutter drill bit) such as shown and described in European Patent No. 0 909 869 B1 to Camco International Inc. and in European Patent No. 0 53 375 B1 to Camco International Inc. U.S. Pat. No. 5,944,127 to Liang et al. and U.S. Pat. No. 6,659,206 to Liang et al. each disclose a rock bit that has a hardfacing deposit. Thus, it can be appreciated that hardfacing is used to extend the service life of drill bits (e.g., a rotary cone rock bit) and other downhole tools used in the oil and gas industry.
Hardfacing can be generally defined as applying a layer of hard, abrasion resistant material to the surface of a less abrasion resistant substrate such as, for example, steel, by plating, welding, spraying or other well-known deposition techniques. Tungsten carbide and its various alloys are sometimes used as hardfacing materials. Hardfacing is typically a mixture of a hard, wear-resistant material embedded in a matrix deposit which is preferably fused with the surface of a substrate by forming metallurgical-type bonds to ensure uniform adherence of the hardfacing to the substrate.
A wide variety of hardfacing materials have been satisfactorily used on drill bits and other downhole tools. Frequently used hardfacing material includes sintered tungsten carbide particles in an alloy steel matrix deposit. Other forms of tungsten carbide particles may include grains of monotungsten carbide (WC), ditungsten carbide (W2C) and/or macrocrystalline tungsten carbide and/or crushed cast tungsten carbide. Further, other metal carbides and/or nitrides, in addition to tungsten carbide, can be used to form a hardfacing deposit. Satisfactory binder materials for the hardfacing may include materials such as cobalt, iron, nickel, alloys of iron, as well as other metallic alloys.
Macrocrystalline tungsten carbide is essentially stoichiometric WC, which is, for the most part, in the form of single crystals. Some large crystals of macrocrystalline tungsten carbide are bicrystals. U.S. Pat. No. 3,379,503 to McKenna, assigned to the assignee of the present patent application, discloses a method of making macrocrystalline tungsten carbide. U.S. Pat. No. 4,834,963 to Terry et al., assigned to the assignee of the present patent application, also discloses a method of making macrocrystalline tungsten carbide.
Crushed sintered cemented (cobalt) macrocrystalline tungsten carbide comprises small particles of tungsten carbide bonded together in a metal matrix. One makes crushed sintered cemented (cobalt) macrocrystalline tungsten carbide through the crushing of previously sintered carbide into blocky and non-blocky shapes wherein this crushed sintered macrocrystalline cemented (cobalt) tungsten carbide. One source of the crushed sintered cemented (cobalt) macrocrystalline tungsten carbide is Kennametal Inc. of Latrobe Pa. 15650 wherein this material is sold under the designation Kenface.
To produce cemented carbide-cobalt pellets, tungsten carbide particles, cobalt powder and a lubricant are mixed together into a mixture. This mixture is pelletized and through a rolling process the mixture of tungsten carbide, cobalt and lubricant ball up into pellets.
Crushed cast tungsten carbide forms two carbides; namely, monotungsten carbide (WC) and ditungsten carbide (W2C). There can be a continuous range of compositions between the monotungsten carbide and the ditungsten carbide. The eutectic mixture is about 4.5 weight percent carbon. Commercially available cast tungsten carbide typically used as a matrix powder generally has a hypoeutectic carbon content of about 4 weight percent. Cast tungsten carbide is typically frozen from the molten state and comminuted to the desired particle size to from the crushed cast tungsten carbide.
One way to apply the hardfacing deposit is to use a hardfacing rod. In this regard, U.S. Pat. No. 5,250,355 to Newman et al. discloses the use of a hardfacing rod to apply the hardfacing deposit. While there may be some variations, generally speaking, a hardfacing rod comprises a hollow tube or rod that contains hard particles. The hard particles are applied to the surface of the article or substrate via welding techniques to form the hardfacing deposit. The hardfacing deposit includes a matrix (e.g., steel or the like) that comes from the substrate itself or from the welding rod or hollow rod. This technique of applying the hardfacing deposit is sometimes referred to as “tube rod welding.”
While these earlier hardfacing compositions, including the hardfacing deposits formed from these compositions using hardfacing rods, have performed in a satisfactory fashion, there remains room for improvement. Thus, it would be desirable to provide an improved hardfacing deposit on an article that overcomes drawbacks associated with the current hardfacing compositions containing hard particles and current hardfacing deposits on an article.
For example, in the current hardfacing compositions containing hard particles, the nature of the particle size distribution of the hard particles results in some drawbacks. More specifically, the particle size distribution in the current hardfacing compositions leaves so-called gaps in the particle size distribution. What this means is that the hardfacing composition does not include hard particles having sizes within certain ranges of particle size distributions. The absence of these particles creates an interruption to the smooth distribution of hard particles across the spectrum of available particle size distributions. Because these gaps (or absences) can lead to certain problems for the hardfacing rod prior to use, as well as for the hardfacing deposit applied to an article, it would desirable to provide an improved hardfacing composition containing hard particles that reduces or eliminates the gaps in the particle size distribution.
One of the drawbacks extant with the presence of gaps in the particle size distribution of the hardfacing composition, and especially with respect to a hardfacing rod, is the possibility of the hard particles to shift their position within the hardfacing rod when jostled or struck or otherwise impacted during handling prior to baking, and such shift in position may occur even after baking as well. This shifting of the hard particles is due to an absence of mechanical support for all of the hard particles, and especially for those particles that are of a size somewhat smaller than the absent particle size distribution that creates the gap in the particle size distribution. Shifting of the hard particles in the hardfacing rod can result in a hardfacing deposit that has an inconsistent microstructure, which can result in less than optimum wear resistance properties. This inconsistency is also thought to create problems during the application of the rod hardfacing itself by having pockets of material deposited instead of the deposition of a homogeneous mixture. Thus, it would be desirable to provide an improved hardfacing composition (including an improved hardfacing rod) containing hard particles that does not present gaps in the particle size distribution so as to reduce or eliminate the shifting of particles due to the jostling of the hardfacing rod as well as to maximize the distribution to increase wear properties.
Another drawback extant with the presence in gaps in the particle size distribution of the hardfacing composition, and especially with a hardfacing rod, is the tendency of the hard particles to migrate to the bottom of the liquid weld pool during the weld pool solidification. As mentioned above, gaps in the particle size distribution of the hard particles leads to a lack of mechanical support for the hard particles. When the weld pool is liquid, this lack of mechanical support allows the hard particles to migrate to the bottom of the weld pool.
The migration of the hard particles results in a non-uniform hardfacing deposit upon the solidification of the weld pool in which the hard particles are generally uniformly distributed throughout the microstructure of the hardfacing deposit. A non-uniform hardfacing deposit leads to uneven wear of the hardfacing deposit during use. Thus, it would be desirable to provide an improved hardfacing composition (including an improved hardfacing rod) containing hard particles that does not present gaps in the particle size distribution, and as a result, there is a reduction or elimination of the migration of the hard particles in the liquid weld pool during the welding operation.
Still another one of the drawbacks extant with the presence in gaps in the particle size distribution of the hardfacing composition, and especially with a hardfacing rod, is the tendency of the deoxidizer (which is a typical component of the hardfacing composition) to segregate in the hardfacing rod. Such segregation could potentially occur when the hardfacing rod is being produced or prior to baking of the hardfacing rod. When the deoxidizer is segregated in the hardfacing composition, there is the tendency to impede the effective release of gases during the welding operation when the weld pool is liquid. By impeding the effective release of the gases, trapped gas pockets form in the weld pool. The presence of these gas pockets could potentially cause the hardfacing deposit to exhibit porosity. Thus, it would be desirable to provide an improved hardfacing composition (including an improved hardfacing rod) that does not present gaps in the particle size distribution, and thereby reduces or eliminates the segregation of deoxidizer so as to reduce or eliminate the presence of trapped gas pockets in the hardfacing deposit.